The present invention relates to a novel and useful reflectometer device used to determine the quality of a mask blank used in the manufacture of electronic chips.
In the recent past, a very substantial advance of scientific techniques has been accomplished permitting the use of extreme ultraviolet (EUV) radiation in lithography process (EUVL) to manufacture electronic chips. The advantage of EULV is that chips of higher density are created, resulting in faster circuits, increased memories, and higher capacity devices, such as computers using such chips. Thus, EUVL has been identified as the most likely method for the next-generation lithography method used in production of electronic chips, since it is possible to operate such manufacturing techniques at the 50 nm node. Recent developments of multiplayer mirror coatings having high reflectivity at EUV wavelengths has heightened such possibility. For example, using the Low Defect Deposition (LDD) system, EULV masks have been produced having ultra clean multi-layers and possessing a high reflectivity, e.g. greater than 60 percent at 13.4 nm.
Needless to say, operating in the 50 nm plateau or node requires a production environment utilizing optical elements of the highest quality. Consequently, each of the projection optics used in the EUVL system must possess diffraction-limited performance at the particular wavelength i.e. 13.4 nm. Thus, stringent requirements on the optical manufacturing of substrates for the reflective masks used in the EUVL process must be maintained. Great strides have been made in the optical field to produce EUV reflective masks possessing surface figure and roughness specifications to a level which allows EUVL production to take place. Further, tolerances on the multi-layer coatings of the EUVL masks are quite stringent. Small errors in coating thickness of EUVL mask also produce errors, similar to figure errors in the optics producing such masks. Coating inaccuracies introduce phase errors into the electromagnetic wavefront, which in turn degrades image quality on the wafer being produced.
The allowable error in the multi-layer coatings of the EUVL masks may be roughly calculated, although more rigorous calculation methods may be employed. However, back-of-the envelope, calculations indicate that coating variations must possess a very narrow range. For example, Rayleigh criteria states that an optical system will have a “diffraction-limited” performance if the wavefront exiting the system does not depart from sphericity by more than one quarter wavelength. In other words, the peak-to-valley deviation from sphericity must be less than λ/4. Thus, an optical system of n elements and figure areas of low spatial frequency add linearly. Each element can therefore contribute no more than λ/4n to the wavefront error. Since a surface error of height creates a wavefront error of 2h, no single surface may possess a figure error of more than λ/8n. Finally, assuming that half of the figure errors in the system are found in the substrates themselves, the error in each multi-layer coating thickness cannot exceed λ/16n. It follows, that a 4-element projection system indicates that the multi-layer coating thickness must be controlled to a tolerance of λ/64. Where λ equals 13.4 nm, thickness variations in a coating (At) must be less than 0.2 nm. Since the total thickness of the multiplayer stack of 40 bi-layers is approximately 40×68=2720 nm, the thickness variation of the coatings must be held to less than one part in 104. Such tolerances have been achieved in manufacturing processes used today.
The attainment of close tolerances on coating thicknesses is due, in part, through coating technology and the availability of methods for measuring the multi-layer coatings at the operating wavelength (13.4 nm). Measuring coating thickness by visible light or mechanical method, such as profilometry, have proven insufficient since the effective period (d-spacing) of the coatings depend strongly on the optical constant in the EUV range. In addition, measurement of the coatings at the operating wavelength is essential for verification that the desired EUV reflectivity has been achieved.
In the past, synchrotron radiation, derived from a bending magnet, has proven advantageous as a source for calibration equipment at EUV wavelengths. Synchrotron radiation possesses high brightness with a smooth continuous spectrum and is slow in variation. Also, synchrotron radiation sources are very clean, by not possessing debris-emitting characteristics found in other sources, such as laser plasma or gas discharge. In addition, synchrotron radiation is usually maintained an operated by personnel who are experts in such technology. The disadvantage of synchrotron radiation is limited accessibility. In the United States of America, only one facility of synchrotron radiation is available at the present time. This means that optics employed in EUVL must be transported to such facility each time a measurement is to be made. Such a process is expensive, time consuming, and further exposes optical components to the risk of damage or contamination during transportation. Also, there is a delay in obtaining results from such measurements which further renders such system inefficient. The location of synchrotron radiation facilities at the EUVL manufacturing facilities would be impracticable since costs to establish such source are astronomical.
An in situ reflectometer device for determining the quality characteristics of mask blank in the EUVL field would be a notable advance in the electronic arts.